1. The Field of the Invention
The present invention relates generally to Distributed Feedback (DFB) lasers, including phase-shifted DFB lasers. More particularly, the present invention is directed towards controlling the facet reflectivity of DFB lasers to improve the yield of lasers having desired characteristics.
2. The Relevant Technology
Distributed Feedback (DFB) lasers are of interest for a variety of applications. In a DFB laser a portion of the vertical optical field distribution interacts with a periodic refractive index change (e.g., a grating). For a uniform grating the resonant DFB laser wavelength, λDFB, is given by the expression:λDFB=2neqΛ/m  (1)where m is the grating order, neq is the average refractive index, and Λ is the grating period.
The grating strength is determined by several factors, including the grating height, grating shape, refractive index step associated with each period of the grating and the distance between the grating and the active layer. Conventionally, a coupling coefficient, κ, can be calculated that is indicative of the grating strength. For a DFB laser having an active region of length, L, the total DFB grating feedback is determined by the κL product. Commonly, a target κL product of between about 0.5 to about 3.0 is desired. The target κL is within a sufficient range to achieve single mode operation over a desired laser power range. For example, for uniform gratings a κL of less than about 1.0 is commonly used whereas for a DFB laser including a phase-shifting section a κL of between 1.0 to 2.0 is commonly employed.
With reference to FIGS. 1A and 1B, a conventional DFB laser fabrication process includes forming a grating 102 on a wafer 100. The grating 102 is oriented parallel to the cleavage planes containing cleaved laser bars 104. This allows a longitudinal mode in a die to interact with the grating. A stripe waveguide 106, which can be, for example, a ridge waveguide or buried heterostructure, is formed perpendicular to the gratings.
As indicated in FIG. 1B, cleaved laser bars 104 commonly have an anti-reflection (AR) coating 110 formed on one of cleaved facets 112. A second facet 114 can have an AR coating or a high reflectivity (HR) coating 108 formed thereon. A saw region 116 is shown between laser stripes 106.
A drawback associated with a conventional laser diode is that the reflectivity associated with facets 112 and 114 can deleteriously affect the DFB laser. The residual reflectivity can reduce the yield of single mode DFB lasers, particularly at high output power levels. This is due, in part, to the fact that the facet reflectivity results in the laser having Fabry-Perot cavity lasing modes that compete with the DFB lasing modes.
In some DFB lasers, such as phase-shifted DFB lasers, it is desirable to use an AR coating, such as that shown in FIG. 1B, to reduce the facet reflection to less than about 1% to suppress the Fabry-Perot modes. AR coatings that can be used include single layer coatings and multi-layer coatings.
So-called “single layer” AR coatings commonly have a coating that is an odd number of quarter-wavelengths thick and that has a refractive index close to the square root of the average refractive index of the laser. The reflectance response of a single layer AR coating is a function of wavelength and has a wavelength at which the reflectivity is a minimum and an associated bandwidth. The bandwidth of a single layer ultra-low reflectivity AR coating is comparatively narrow, which can make it difficult to achieve a low reflectivity at the lasing wavelength. Consequently, single layer AR coatings used in DFB lasers commonly have a reflectivity of about 1% or higher.
Alternatively, a multi-layer AR coating can be used, such as multilayer coatings having quarter wavelength and half wavelength thick layers with the refractive index of each layer selected to produce a desired reflectance response. An advantage of a multi-layer AR coating is that the bandwidth of an ultra-low reflectivity multi-layer AR coating can be broader than a single layer ultra-low reflectivity AR coating. However, a multilayer AR coating typically requires a greater number of layers to produce a low reflectivity, increasing its cost.
Unfortunately, a multilayer AR coating having a low reflectivity typically requires precise control of the refractive index and thickness of each layer, resulting in large lot-to-lot variances. Also, a thick multilayer AR coating can have or induce a mechanical stress that is deleterious to the long-term reliability of the laser. As a result of the drawbacks of single-layer and multi-layer AR coatings, DFB lasers commonly have a facet reflectivity greater than 1% which results in a lower yield than desired.